In the art of power tools for tightening threaded fasteners many of the uses of such tools require relatively precise control of the final torque exerted on the fastener when making up the joint or connection. A considerable range of joint stiffness as well as other factors must be accommodated while yet providing accurate control of the torque applied to the fastener itself. The solution to this problem has been attempted by the development of numerous types of control devices for controlling the torque output of power wrenches, nutsetters and the like.
It has been determined that in tightening fasteners in joints which exhibit a very stiff or hard makeup, that is a joint in which the fastener reaches its final desired torque very rapidly after a relatively torque-free rundown operation, that overtightening of the fastener is difficult to avoid because of the rotational inertia of the tool rotating parts and because of the elapsed time required to effect deenergization of the tool motor by prior art control mechanisms.
Various control devices including types which sense changes in the rotative speed of the drive spindle and types which sense changes in pressure of the working fluid in pneumatic power tools have been only partially successful in meeting the need for precise torque control. Many of the prior art mechanisms which sense the reaction torque on the tool are slow operating or, in the case of electrical sensors, are expensive and somewhat unreliable when used in the typical operating environment of power tools used for assembly operations.